The first Wideband Code-Division Multiple Access (W-CDMA) networks, specified in Release 99 of the 3rd-generation mobile system standards produced by the 3rd-Generation Partnership Project (3GPP), provided significantly improved data rates compared to predecessor GSM/GPRS systems. Data rates of up to 384 kilobits per second became possible, but the number of users that could be served simultaneously was low, due at least partly to the circuit-switched architecture of the system. Furthermore, these maximum data rates, although much better than previously available, still did not compete favorably with data rates achievable over fixed lines.
In order to improve data services quality, High-Speed Packet Access (HSPA) functionality was added in Releases 5 and 6 of the 3GPP specifications. HSPA, through its downlink component (High-Speed Downlink Packet Access, or HSDPA), can provide packet-switched connections to several simultaneously active users. Release 5 of HSPA, using QPSK and 16-QAM modulation schemes, offered downlink data rates of 1.8-14.4 megabits/second. Later extensions to the standards added specifications for Multiple-Input/Multiple-Output (MIMO) technologies as well as a 64-QAM modulation scheme, allowing peak rates up to 42 megabits/second.
HSDPA downlink data is generally transmitted from a single cell, although it may involve transmission from multiple antennas when MIMO is employed. The original design of HSDPA was driven by a desire to improve the available peak rates for users with good reception conditions, when network loading conditions permit. A mobile terminal is served by a single serving cell, and control signaling is provided over a single radio link. With favorable signaling and loading conditions, interference from neighboring cells is not a major impediment. However, high-rate coverage in more fully loaded systems and in interference-limited scenarios was not prioritized at the time HSDPA was first conceived and designed.
Because the system design for HSDPA was historically focused on maximal data throughput under benign conditions, mobile stations operating under less favorable signal conditions do not benefit as much from the system improvements. For instance, downlink data throughput at cell edges is typically not improved with higher-order modulation schemes or MIMO schemes, since interference from neighboring cells limits the achievable signal-to-interference ratio (SIR) at the mobile station. Advanced receivers, such as those employing Generalized Rake (G-Rake) technology, are capable of suppressing some of the inter-cell interference, but the interference from neighboring cells may not be completely removed since noise enhancement effects limit the interference suppression capabilities of practical receiver structures. In general, link-level improvements from more sophisticated receiver processing alone are insufficient to eliminate the negative effects of inter-cell interference.
Accordingly, there is a need for improved network configurations and resource allocation techniques, to provide higher downlink data rates to mobile terminals situated near cell edges while maintaining high levels of data throughput on a system level.